The Baltic Sea has been affected by eutrophication for several decades with the result of a gradual increase of oxygen minimum zones at the sea floor and release of phosphate from sediments. This has generated a decreasing inorganic Nitrogen:Phosphorus (NP) ratio and excess phosphate remaining after the spring bloom. In the present study, a 17-days experiment was performed in June 2021 at the Tvärminne Research Station (Gulf of Finland, Northern Baltic Sea) where the effects of the decreasing ratio of dissolved inorganic Nitrogen and Phosphorus and a thunderstorm were investigated on plankton microbial communities in 12 in-situ mesocosms via High-Troughput amplicon sequencing. 500 ml of water were filtered on two different sterile polycarbonate filters (Whatman® Nuclepore™ Track-Etched Membranes diam. 25 mm, pore size 0.2 µm). Filters were placed in sterile cryogenic vials before immediate flash-freezing in liquid nitrogen and stored at -80? until being processed for nucleic acid extraction. DNA was extracted using a modified protocol from Nercessian et al. 2005. PCR amplification was performed with the primers 341F (5'-CCTAYGGGRBGCASCAG) and 806R (5'-GGACTACNNGGGTATCTAAT) covering the V3–V4 region of the 16S rRNA gene, and with the primers ITS4ngsF (5'-GCATATCAATAAGCGSAGGA-3') and LF402R (5'-TTCCCTTTYARCAATTTCAC-3') covering the D1 region of the LSU (modified after Tedersoo et al. 2015). Sequences were generated on a MiSeq Illumina platform in a 2x300bp paired-end run using V3 Chemistry (Illumina) in mixed orientation. This study was funded by: AQUACOSM - Network of Leading European AQUAtic MesoCOSM Facilities Connecting Mountains to Oceans from the Arctic to the Mediterranean Horizon 2020 - INFRIA (Award number: 731065).